In the recent multi-functional electronic appliances, back-lighting is increasingly introduced to illuminate the display panels or LCDs from behind, so that an operator can easily recognize the display to operate the appliance even in the darkness. EL element is popular means used for such back-lighting.
A conventional EL element is described referring to FIG. 3 and FIG. 4.
FIG. 3 is a cross sectional view of a conventional EL element. Referring to FIG. 3, a light transmitting electrode layer 2 of indium tin oxide (ITO) is formed by sputtering or an electron beam deposition on the whole surface of a polyethylene terephthalate or the like light transmitting insulating film.
Provided on the ITO are; a light emitting layer 5 comprising a binder 3 such as fluorocarbon rubber, cyano-resin or the like synthetic resin of high dielectric constant and phosphor particles 4 such as zinc sulfide or the like dispersed therein, a dielectric layer 6 of high dielectric constant resin containing barium titanate or the like high dielectric constant inorganic filler dispersed therein, a back electrode layer 7 of silver or carbon dispersed in resin system, and an insulating layer 8 formed of an epoxy resin, polyester resin or the like materials. Each of the layers is provided overlaid one after another by a printing method. The conventional EL elements are thus manufactured.
An EL element of the above configuration mounted on an electronic appliance is supplied with an AC voltage on the electrode layer 2 and the electrode layer 7 from a circuit of the electronic appliance (not shown), then the phosphor 4 in the light emitting layer 5 emits light to illuminate display panel, LCD and the like of the appliance from the behind. In this way, the displays or the operating panels can be easily recognized even in the dark environment.
In order to efficiently excite the phosphor 4 for obtaining a high brightness, the resin of the dielectric layer 6 is filled with a high dielectric constant inorganic filler to the highest possible extent in order to raise the dielectric constant. Meanwhile, the light emitting layer 5 is set to have a low dielectric constant so that AC electric fields concentrate on the light emitting layer 5. As the result, most of the AC voltage applied between the electrode layer 2 and the electrode layer 7 concentrate to the light emitting layer 5.
If the EL element is put into operation in a high humidity environment, a local discharge sometimes occurs in the resin 3 of the light emitting layer 5 by the humidity and the voltage, and the carbonized resin 3 results in a so-called black spot, which impairs the illumination.
The assumed reason is that; by the effect of the humidity and the voltage, zinc ion melts out of the phosphor 4 in the light emitting layer 5, which decreases insulating property of the resin 3 containing moisture. For preventing the above phenomenon to occur, the phosphor 4 of zinc sulfide or the like is provided with a moisture barrier layer 4A formed of metal oxide such as aluminum oxide, titanium oxide, silicon dioxide and the like, or formed of aluminum nitride and the like.
In the conventional EL elements, however, if a plurality of phosphor particles 4 coagulate as shown in FIG. 4(a), the contacting area 9 between the phosphor particles 4 can be left uncovered by the moisture barrier layer 4A of titanium oxide and the like. In other case, when the phosphor particles 4 coated with the moisture barrier layer 4A are stirred in a paste-state where resin 3 is mixed with a solvent, or when the paste is transferred to other place, the moisture barrier layer 4A can be damaged and the phosphor 4 is exposed, as illustrated in FIG. 4(b), as a result of collision among the phosphor particles 4. Under such situation, zinc ion dissolves out from the phosphor particles 4, which readily deteriorates insulating property of the light emitting layer 5 in high humidity environment, causing the problem of black spot.
Furthermore, in a case where the moisture barrier layer 4A has been formed using aluminum nitride, instead of metal oxide, the aluminum nitride can decompose in a high humidity environment by hydrolysis to generate ammonium ion, even if the covering is perfect. The insulating property with the resin 3 of the light emitting layer 5 can be readily impaired.
The present invention addresses the above-described drawbacks with the conventional EL elements, and aims to provide an EL element in which the insulating property of light emitting layer is well maintained even in a high humidity environment and generation of the black spot is suppressed, even if the moisture barrier layer covering a phosphor was imperfect, or the moisture barrier layer was formed using a easily hydrolyzed material such as aluminum nitride and the like.